The invention relates to a mixing and kneading machine for continual compounding including a screw shaft rotating in a casing and simultaneously moving axially translationally. The invention also relates to a method of implementing continual compounding by means of a mixing and kneading machine engineered.
Mixing and kneading machines of the kind presently involved are employed particularly for compounding bulk-flowable (powder, granulate, flakes, etc.), plastic and/or pasty masses.
The working member of the mixing and kneading machine is usually configured as a so-called screw shaft which forwards the material for processing axially.
In conventional mixing and kneading machines the working member merely produces a rotational motion. In addition, mixing and kneading machines are also known in which the working member rotates whilst at the same time moving translationally. The motion profile of the working member is characterized particularly by the main shaft executing a sinusoidal motion overlying the rotation. This motion profile permits casing-mounting such fitted items as kneader pins or kneader teeth. For this purpose the curved vane of the screw shaft is discontinued to form discrete kneader or screw vanes. The screw vanes disposed on the main shaft and the casing-mounted fitted items interact in thus creating the desired shear/mixing and kneading functions in the various processing zones. Such mixing and kneading machines of the last-mentioned kind are known to persons skilled in the art under the trade name Buss Ko Kneaders®.
One such Buss Ko Kneader® is described in Swiss patent CH 528 294, it comprising a casing in which a quill shaft is mounted to simultaneously combine its rotary and reciprocating motion. The shaft is engineered with four screw vanes or a multiple thereof, each cooperating with kneader teeth lining the casing. In a first example embodiment the inner diameter of the casing is 200 mm, it being 400 mm in a second example embodiment and 600 mm in a third example embodiment, resulting in the outer diameter of the shaft being a tight 200 mm in the first example, a tight 400 mm in the second and a tight 600 mm in the third example. For all casing diameters the effective length of the machine is given as 1390 mm, corresponding to a ratio of processing space length (Pl) to screw shaft outer diameter (Da) ranging from approx 2.3 to 7. The difference between the diameter of the casing and the diameter of the shaft core is for all sizes 70 mm. The ratio of screw shaft outer diameter (Da) to screw shaft inner diameter (Di) thus ranges from approx 1.13 to 1.54. The number of axial movements of the screw shaft is proportional to the casing diameter whilst shaft rpm is selected inversely proportional to the casing diameter. For a casing diameter of 400 min the screw shaft performs two axial movements per revolution, whilst for a casing diameter of 600 mm it performs three axial movements per revolution. The geometric core parameters (a, b and d, e resp.) are selected for this machine so that no matter what its size its screw channel depth (s) is always the same: s=(b−a)/2=(e−d)/2. This results in the ratio numbers for the surface and volume of each size being formed the same. The significance of this is that scaling the size up or down always needs to be achieved via the available surface. This is why this machine is only suitable for methods and processing defined exclusively by surface actions (e.g. heat exchange). On top of this, with a machine engineered as such, only relatively small amounts of material can be processed per unit of time, because scaling up or down is possible maximally over the square ratio of the screw outer diameter.
It is on the basis of this prior art that future methods and machines need to ensure that the methods and the scaling up/down associated therewith are operated as near as possible to the volume actions involved. This calls for the geometric ratios needing to be selected so that—in the terms as recited above—at least the ratios b/a and e/d, but preferably all other values too within the series result in more or less the same values in thus making it possible to scale up or down over the cubic ratio of the screw outer diameter.
For certain areas of application such as, for instance, in compounding an anodic mass in the production of electrodes, such as anodes, for the aluminum industry there is a need for sustainably boosting the output of the machine without having to engineer the machine substantially larger which, in addition to adding to the costs, also involves other drawbacks; for example, the mechanical stress (both static and dynamic) being increased out of all proportion, differences in the thermal expansion between the screw shaft and casing along with an unfavorable change in the surface to volume ratio. Since in obtaining aluminum by means of electrolysis each anode is consumed due to the oxygen resulting in the process, the anode needs to be replaced new every time. Producing aluminum is usually done by means of fused salt electrolysis of aluminum oxide by the cryolite clay process in which aluminum oxide is dissolved in a cryolite melt to lower the melting point. At this time, the annual demand for electrode masses in the aluminum industry is estimated to be around 13 million tons per year worldwide.
Mixing and kneading machines for compounding an electrode mass are known in which the screw diameter is 700 mm, it being particularly the screw diameter that dictates the material thruput in the production of the electrode mass which is substantially compounded from coke and pitch, amounting to 55,000 kg/h with the largest shaft diameters. Depending on the size of the mixing and kneading machine the screw shafts of known machines are run at speeds ranging from 20 to 60 rpm.